Class III anti-arrhythmic agents may be categorized as having the ability to markedly prolong dog Purkinje fiber action potential duration without producing significant changes in maximal upstroke velocity. Unlike Class I anti-arrhythmic agents, a pure Class III agent displays no effects on cardiac sodium channels. The electrophysiologic properties of a compound defining a Class III activity profile are observed in vivo as negligible effects on atrial, ventricular and H-V conduction line while producing a marked increase (greater than 20 percent) in both the atrial and ventricular refractory period. In contrast, Class I agents will demonstrate a marked slowing of ventricular conduction velocity without significant changes in the refractory period. Recent reviews of these agents are by Bexton et al., Pharmac. Ther. 17, 315-55 (1982); Vaughan-Williams, J. Clin. Pharmacol. 24,129-47 (1984) and Thomis et al., Ann. Rep. Med. Chem. 18, 99-108 (1983).